Diodes

The part that only lets current through one way — charges about 0.7 V for the privilege, holds a fixed voltage when you push it backwards hard enough, and can be talked into storing a number.

1. 16What a diode is6 minA diode is a part that lets current through one way and blocks it the other. That one sentence is most of what you need — and the four circuits it turns up in are the reason there is a diode on nearly every board you will ever own.
2. 17Inside a diode7 minNothing inside a diode moves, and it still behaves like a valve. What is in there is two pieces of silicon treated differently, and a stripe between them a few ten-thousandths of a millimetre wide that current can only cross in one direction.
3. 18The forward voltage7 minA conducting diode takes about 0.7 V and gives you the rest. That subtraction is why a battery project can stop working long before the battery is flat, why an LED needs a resistor beside it, and why nobody ever sets a diode's current by choosing a voltage.
4. 19Which way round a diode goes6 minThe stripe is the cathode, the bar in the symbol is the same end, and the long leg of an LED is the other one. Two minutes with a multimeter proves it on any part in your drawer — and finds the failed diode that made a board stop working.
5. 20The Zener diode7 minEvery diode has a reverse voltage that destroys it. A Zener has one that is printed on the label, held to a few per cent, and survivable — so you can run the part backwards on purpose and keep the voltage it sits at.
6. 21A Zener as a regulator8 minOne resistor and one Zener make a fixed rail out of a supply that wanders. Whether it works comes down to two sums — the lowest input with the biggest load, and the highest input with no load at all — and the resistor has to satisfy both at once.
7. 22Diodes and flyback7 minA diode is a one-way valve, and most of the time that is all you need to know. The exception is the diode fitted across a relay or a motor, which does nothing at all until the moment it saves the transistor beside it.
8. 23A ROM made of diodes8 minA diode cannot amplify anything, but it can decide. With nothing but diodes and resistors you can build an OR gate, an AND gate, and a read-only memory whose contents are literally which crossings someone soldered a part into.